The technology of digital data storage using optical memory devices has advanced rapidly since its inception. Commercial uses at present range from compact discs (CDs) which provide high quality audio reproduction to computer memories which provide extremely large yet compact storage capacity. As applied to the requirements of computer mass storage, optical disks have been categorized according to the ease with which data can be written onto them. Optical read-only memories (OROMs) such as compact discs (CD-ROMs) have data written on to them before they leave the factory; write-once read-many (WORM) disks allow the user to write data onto them once and read that data indefinitely; erasable optical disks permit the user to write and read data with the same flexibility as magnetic storage media such as floppy and hard disks. Jones et al, Encyclopedia of Polymer Science and Engineering, Supplement Vol., Second Ed., John Wiley & Sons, New York (1989), pp. 554-567 present a concise discussion of the technology of optical data storage.
One of the reasons that optical storage media have increased in popularity is the very large capacity they provide in a small volume. Information is usually stored on the disk in the form of pits in the recording layer. The pits can consist of holes, which are arranged in tracks, in the recording layer. The holes are usually formed by an intense laser beam focused onto the layer which is formed of a material which absorbs the laser radiation and ablates or melts as a result of heating caused by the energy absorbed. The intensity of the light reflected from the disk is modulated by the presence or absence of the pits. The recording layer may be composed of tellurium alloys, bubble-forming materials, multilayer optical cavities, colloids, microtextured absorbers or organic materials, such as those described by Kalyanaraman et al. in Functional Polymers, edited by D. Bergbreiter and C. Martin, Plenum Publishing, New York, pp. 173-191 (1989). The pits are written by a relatively high power laser beam, for example, 10-30 mW, while they are read by a low power laser beam, typically 0.5-1 Mw. Both writing and reading are usually performed by a semiconductor laser, such as those of gallium arsenide.
The light-sensitive material in the recording layer (also called the information layer) normally comprises a material which exhibits a change in its physical characteristics, such a melting or evaporating, to produce a hole or pit, whenever the writing beam is focused thereon. The intensity of the writing beam is modulated according to the information to be recorded, and therefore, is alternately greater than or less than a predetermined threshold, at which the melting or evaporation occurs in the recording layer. A sequence of spaced holes or pits representing the information is thereby formed in the layer. In the reading process, the read beam is reflected off the recorded and unrecorded areas of the disk with different reflectivities. The term ,contrast, is related to and signifies this difference in reflectivity. The difference in reflectivity or reflectivity change is read by the detector and converted back into the information. Generally, in a pit forming process, the recorded areas have lower reflectivities than the unrecorded areas and give rise to negative contrast. On the other hand, if the reflectivity during writing increases in the recorded areas it would give rise to positive contrast. Since, in the positive contrast mode, reflectivity increases in the recorded areas, the laser power needed to write is generally lower than that in a negative contrast mechanism. In other words, the sensitivity of the medium is higher. Also, due to the use of lower energy, the medium may have longer life.
The use of azaannulene compounds such as phthalocyanines, naphthalocyanines, phenanthracyanines, porphyrins, anthracyanines, and the like, as recording layers in optical data storage has been well documented. Some patents in this regard are U.S. Pat. Nos. 4,241,355; 4,298,971; 4,458,004; 4,492,750 and 4,725,525, and EP 0,296,876. As recording layer materials, azaannulene compounds perform generally by a negative contrast process.
U.S Pat. No. 4,946,762 discloses phthalocyanine compounds that undergo non-ablative recording by phase transformation resulting from thermally altering the media during the recording. These thermally altered areas, sometimes described in that patent as `spots`, show an increased reflectivity in the recorded areas.
U.S. patent application Ser. No. 462,680, filed Jan. 9, 1990, discloses azaannulene compounds which show a positive reflectivity change of 20-40% during marking without effecting a visually or optically detectable deformation in the information layer.
U.S. patent application Ser. No. 333,523, filed Apr. 4, 1989, discloses an optical medium comprising a mixture of tetraazaporphyrin dye constituents.
As the technology of optical recording media becomes more sophisticated, the need and desire for more sensitive recording media, thereby requiring less powerful and/or smaller lasers, will become greater. Media requiring less power and less time in which to record information, would be a great advance in the technology. One way to achieve recording at less powers is to have a significant enhancement of the positive contrast which may, in turn, enable the reading of the spots much easier.
Accordingly, it is an object of the present invention to provide a system which can record information on an optical recording medium using less power than is typically used in optical information recording.
Another object of this invention is to provide a process whereby an enhanced positive contrast is produced on optical recording resulting in an easier reading of the information.
It is yet another object of this invention to provide a system which provides enhanced positive contrast reflectivity change in the recorded areas during optical recording.
These and other objects of the present invention will become apparent to the skilled artisan upon a review of the following specification, and the claims.